The present invention relates to a color cathode-ray tube having a shadow mask.
In general, a color cathode-ray tube (CRT) has a vacuum envelope. The vacuum envelope comprises a substantially rectangular panel, which has a substantially rectangular effective portion with a curved surface and a skirt portion provided on a peripheral area of the effective portion, and a funnel coupled to the skirt portion of the panel. A phosphor screen is formed on an inner surface of the effective portion of the panel. The phosphor screen includes black non-light emission layers and striped tricolor phosphor layers interposed among the non-light-emission layers. A substantially rectangular shadow mask is disposed inside the phosphor screen so as to face the phosphor screen. On the other hand, an electron gun for emitting three electron beams is disposed within a neck of the funnel.
In the color CRT, three electron beams emitted from the electron gun are deflected by magnetic fields generated by a deflecting device mounted on an outer surface of the funnel. The deflected beams are horizontally and vertically scanned over the phosphor screen through the shadow mask. Thus a color image is displayed.
The shadow mask serves to selectively pass the three electron beams from the electron gun in association with the tricolor phosphor layers. The shadow mask comprises a substantially rectangular mask body and a substantially rectangular mask frame. The mask body has a curved effective surface opposed to the phosphor screen, and a number of electron beam passage apertures are formed in the curved effective surface. The mask frame is attached to a peripheral portion of the mask body. The shadow mask is detachably supported within the panel by engaging wedge-shaped elastic support members attached to the respective corner portions of the mask frame with stud pins fixed to the respective corner portions of the skirt portion of the panel.
In general, in order to display an image without color blur on the phosphor screen of the color CRT, it is necessary to selectively pass the three electron beams through the electron beam passage apertures in the mask body so that the electron beams may exactly land on the associated tricolor phosphor layers. For this purpose, it is also necessary to exactly determine the positional relationship between the panel and the shadow mask and, in particular, to maintain a distance (value q) between the inner surface of the effective portion of the panel and the effective surface of the mask body within a predetermined tolerable range.
In modern color CRTs, in order to enhance visibility and reduce reflection of external light, it is desired that the curvature of the outer surface of the effective portion of the panel be decreased (i.e. the radius curvature be increased) to a level of a substantial flat plane. In the case of this type of panel, the curvature of the inner surface of the effective portion needs to be decreased from the standpoint of manufacture of the vacuum envelope and visibility. In addition, with an increase in radius of curvature of the inner surface of the effective portion, the curvature of the effective surface of the mask body also needs to be decreased to achieve exact beam landing.
If the curvature of the effective surface of the mask body is decreased, however, the curved-surface retention strength decreases and the mask body may deform in the manufacturing process of the color CRT.
In a color CRT, because of the operational principle, the amount of portions of electron beams, which pass through the electron beam passage apertures in the shadow mask and reach the phosphor screen, is less than 1/3 of the entire amount of electron beams emitted from the electron gun. The other portions of electron beams collide with those parts of the shadow mask which are other than the electron beam passage apertures and are converted to thermal energy to heat the shadow mask. Consequently, the shadow mask thermally expands and doming towards the phosphor screen occurs.
If the distance between the phosphor screen and the effective surface of the mask body decreases to a level out of the predetermined permissible range due to the doming, an error will occur in positions of beam landing on the tricolor phosphor layers and color purity will deteriorate. The magnitude of displacement of beam landing due to thermal expansion of the shadow mask varies greatly depending on the luminance of an image pattern and the duration of the pattern. In particular, when a high-luminance image pattern is locally displayed, local doming will occur and a great displacement of beam landing will locally occur in a short time.
It is well known that where the curvature of a mask body is decreased, local doming will increase. In this case, too, local displacement of beam landing will occur and the color purity deteriorate.
Japanese Patent Application No. 10-199417 discloses a technique for correcting a displacement of beam landing in a case where the curvature of the mask body is decreased, as described above. According to this technique, a color CRT has a flattened outer surface of a panel whose curvature is substantially zero. The color CRT also has a shadow mask with a mask body whose effective surface has such a cylindrically curved shape that the curvature in its long axis is substantially zero and that in its short axis is fixed at a constant value.
With use of such a curved surface, the problem of doming of the shadow mask can almost be solved. However, the curved-surface retention strength of this shadow mask is inadequate, and the problem of deterioration in color purity due to deformation of the shadow mask remains to be solved.